1. Field of the Invention
The present invention is related to the field of high pressure fuel injectors for internal combustion engines of the type having a plunger assembly, with a plurality of plungers, that is mounted within a central bore within the body of the fuel injector for reciprocal movement. More specifically, the invention relates to such a fuel injector where the plunger assembly has an upper plunger and a lower plunger mounted for reciprocal motion within the central bore and a variable volume injection chamber in said lower end of the central bore between the injection orifice and a bottom end of the lower plunger.
2. Description of Related Art
A fuel injector of the initially mentioned type is known, for example, from U.S. Pat. Nos. 4,721,247 and 4,986,472 (which are owned by the assignee of this application). While an improvement over then existing fuel injections systems, such fuel injectors have a return spring which serves to draw the lower injection plunger upwardly into engagement with an intermediate plunger to force these plungers and an upper plunger together after completion of an injection cycle until metering and timing has commenced for the next cycle, and thereby, establishing a preload force which must be overcome to meter timing fluid into the timing chamber (between the upper and intermediate plungers) to vary the advancement of injection timing. Thus, even though, at times, there is relative movement between the plungers of the plunger assembly, during retraction of the plunger assembly, the lower plunger executes the same retraction stroke as the upper plunger. As a result, a considerable quantity of air is drawn from the combustion chamber of the engine, through the open nozzle, into the fuel metering, variable volume injection chamber during the retraction stroke of the plunger assembly. This fuel-laden air, when compressed during the next injection stroke, can detonate and lead to premature detonation of the fuel which has been metered into the injection chamber, as well. In fact, during development of this invention an attempt was made to use a rigid stop; however, this stop showed extreme wear and cracking after a few hours of operation since the plunger was hitting the stop at near the maximum plunger retraction velocity.
Of course, the use of abutments to limit the stroke of a reciprocable member is commonly known in a wide variety of fields too numerous to mention. However, in the context of a plunger assembly of a high pressure fuel injector, repeated high speed metal-to-metal contact between a plunger and an injector body component, in which the momentum of the plunger must be absorbed by the injector body component, is undesirable from a number of standpoints including wear, noise, etc.